The high temperature resistance and chemically non-reactive properties of ceramics make ceramic components a desirable replacement for traditionally metal components. A particular example of this can be found in the turbomachinery field where ceramic turbine wheels have begun to replace metallic turbine wheels within turbochargers for motor vehicles. Ceramic turbine wheels can operate at higher temperatures, are lower in weight and inertia, and are chemically non-reactive, thus making them superior to their metallic counterparts.
However, ceramics have certain properties which make it difficult to support a ceramic shaft, which for example may extend from a ceramic turbine wheel, with traditional bearing systems such as lubricated ball bearings or journal bearings. In addition, known ceramics capable of operating at very high temperatures are relatively good conductors of heat. When the turbine wheel is formed integral with the shaft, heat is conducted from the turbine wheel along the shaft causing bearing surfaces to become very hot. When these shafts are supported by oil lubricated ball or journal bearings, the high temperature of the shaft may cause coking of the oil. This coking of the bearings will rapidly lead to failure of the turbomachine.
In certain applications, fluid film gas bearings utilizing metal foils to support a shaft on a hydrodynamic film of gas have replaced ball bearings and journal bearings. Example of these types of process fluid bearings, include U.S. Pat. Nos.: 3,893,733; 3,957,317; 4,153,315; 4,178,046, 4,195,395; 4,475,824; 4,502,795; 4,621,930; and 4,624,583.
In the operation of these fluid film gas bearings, at startup and rundown, there is actual contact between the foils and the bearing surfaces. In order to lower startup friction and prevent wearing of the contact surfaces, the foils may be coated with a lubricant material such as a fluorinated hydrocarbon polymer, graphite, or molydenum disulfide. Examples of patents specifically directed to foil coatings are U.S. Pat. Nos.: 3,677,612; 4,005,914; 4,178,046; 4,435,839; and 4,459,047.
These types of fluid bearings would be an ideal match for ceramic shafts except for soak back heating of the shaft from the turbine wheel. The various lubricant materials in use as well as the metal foils are susceptible to damage at elevated temperatures. Heretofore an acceptable bearing support system for a ceramic shaft which is exposed to high temperatures has not been available.